Actuators are frequently used as mechanisms to introduce motion or control motion. It is operated by a source of energy, typically electric current, hydraulic fluid pressure, or pneumatic fluid pressure, and converts that energy into motion of a target mechanism, such as into movement of a closure element of a control valve.
A control valve is generally used for a continuous control of a liquid or gas flow in different pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam. The control valve is usually connected with an actuator, which moves the closing element of the valve to a desired position between fully open and fully closed positions. The actuator may be a pneumatic cylinder-piston device, for example. The actuator, for its part, is usually controlled by a valve positioner, also called as a valve controller, which controls the position of the closing element of the control valve and thus the material flow in the process according to a control signal from the process controller.
Valves generally applied in the industry are often operated by means of pneumatic actuators. These actuators convert a pneumatic into valve stem motion by pressure acting on a diaphragm or piston connected to the stem. The actuators can be either single-acting or double-acting. With the single-acting devices, movement in the opposite direction is effected by a spring, compressed air working against the spring. When air pressure closes the valve and spring action opens the valve, the actuator is termed direct acting. When air pressure opens the valve and spring action closes the valve, the actuator is termed reverse acting. Double-acting actuators have air supplied to both sides of the diaphragm or the piston. The differential pressure across the diaphragm or the piston positions the valve stem. Automatic operation is provided when the pneumatic signals are automatically controlled by circuitry. Semi-automatic operation is provided by manual switches in the circuitry to the air control valves. Also hydraulic actuators may be employed for positioning of the valve similar to the pneumatic actuators, but now a hydraulic fluid is used instead of air or a pneumatic fluid.
A valve positioner can typically receive control commands over a digital fieldbus or as an analog 4 . . . 20 mA control signal. Highway Addressable Remote Transducer (HART) protocols allow transmission of digital data together with a conventional 4 to 20 mA analog signal. Other examples of fieldbuses are Fieldbus and Profibus. Typically all electric power to a positioner is taken from the fieldbus or the 4 . . . 20 mA control signal. A separate electric power supply to a positioner is not desired, because this would require a separate cabling. A positioner may include an electronic unit having an electrical control output and a pneumatic or hydraulic unit that takes in the electrical control signal and converts it to a corresponding fluid pressure output to an actuator. This is often referred to as a current-to-pressure (UP) conversion. The pneumatic or hydraulic unit may comprise a prestage and an output stage. Because the electric power available from the fieldbus or analog current loop is very limited, the prestage may first convert the electrical control signal into a small pilot fluid pressure which is sufficient to control the output stage. The output stage is connected to a supply fluid pressure and amplifies the small pilot pressure signal into a larger fluid pressure output signal used by the actuator. The output stage is often referred to as a pressure amplifier, a pressure booster, or a pressure relay.
Pneumatic output stages used in positioners can coarsely be grouped into spool valve assemblies and poppet valve assemblies. A simplified design example of a 5/3 spool valve (5 ports/3 states) for controlling a double-action actuator is illustrated in FIG. 1A and the corresponding schematic symbol FIG. 1B. In an output stage of a spool valve type the only moving part is a spool 6 which moves within a central bore in a valve body 7 and controls an air flow from a supply pressure port 1 to the actuator ports 2, 4, and from the actuator ports 2,4 to exhaust ports 3 and 5. Due to the structure of the spool valve, there is always a supply air leakage through the valve. The strict tolerances make manufacturing techniques of spool valves very demanding. Generally, the output stage of a spool valve type is not robust to changes in operating environment and in manufacturing.
An output stage with a poppet valve design has got higher number of moving parts than a spool valve. However, the larger tolerances and clearances allowed for the spool valve parts make it possible to utilize an economical mass production and modern manufacturing techniques. A simplified design example of a conventional 4/2 poppet valve (4 ports/2 states) for controlling a double-action actuator is illustrated in FIG. 1C and the corresponding schematic symbol in FIG. 1 D. As can be seen, in a conventional poppet valve assembly two separate poppet valves 8 and 9 are required to control an air flow from a supply pressure port 1 to the actuator ports 2,4, and from the actuator ports 2,4 to the exhaust port 3. In the conventional output stage illustrated in FIG. 1C the controllability with a single pilot pressure is poor, since the movements of the poppet valves 8 and 9 are not mechanically connected to each other. U.S. Pat. No. 6,276,385 discloses an output stage wherein the movement of poppet valves are together by an actuation beam to move in unison, but in opposing directions. The actuation beam is a rocker arm rotating upon a central pivot. The movement of poppet valves is now synchronized.
Both in the conventional output stage illustrated in FIG. 1C and in the output stage of U.S. Pat. No. 6,276,385 the control of the poppet valves requires very large forces to overcome the pressure forces. The threshold force required to open a poppet valve becomes large and introduces a significant point of discontinuation within the control region. This characteristic of prior art output stages of poppet valve type makes the control of the output stage significantly more difficult.
Examples of 3/2 output stages (3 ports/2 states) of poppet valve type for a single-action actuator are disclosed in U.S. Pat. Nos. 6,276,385, 6,957,127, 8,522,818, 7,458,310, and 5,261,458.